The document is a report on a study tour to OPTCL (Odisha Power Transmission Corporation Limited) submitted by 4 students. It provides an overview of the tour activities including an interactive classroom session covering electrical power transmission and distribution systems. It then describes the field visit where students observed and learned about various transmission equipment such as capacitive voltage transformers, current transformers, wave traps, isolators, circuit breakers and surge arresters.

1. STUDY TOUR
TO
OPTCL
SUBMITTED BY:-
1. SOUMYA RANJAN DAS (39)
2. SATYABRATA NAYAK (43)
3. SITUPARNA SAHU (54)
4. BADRI NARAYAN SAHU (55)

2. INDEX
SL.NO. TOPIC PAGE NO.
1 ACKNOWLEDGEMENT 1
2 OBJECTIVE 2
3 REPORT 3
i ELECTRICAL POWER TRANSMISSION 4
ii POWER SUPPLY SYSTEM 4
iii OVERHEAD TRANSMISSION 5
iv EFFECT OF LIGHTNING 5
v CAPACITIVE VOLTAGE TRANSFORMER 6
vi CURRENT TRANSFORMER 6
vii WAVE TRAP 7
viii ISOLATER 8
ix CIRCUIT BREAKER 9
x PUPOSE OF GUARD RINGIN
10
SUSPENSION INSULATORS
xi SURGE ARRESTER 10
xii LIGHTNING ARRESTER 11

3. ACKNOWLEDGEMENT
In the first instance, we would like to thanks our course coordinator Ms. NIVEDITA
PATI who had planned such an interesting and useful study tour and also guided us thoroughly
through the entire tour.
We also thanks Mr. KANHU CHARAN BISOYI of OPTCL who gave us all the
required knowledge that was required during the field visit.

4. OBJECTIVE
The study tour was arranged by our EEE department to get proper exposure of the real
life problems and challenges faced in the transmission and distribution system and also to gain a
proper practical knowledge about the transmission and distribution system.

5. STUDY TOUR TO OPTCL
Bhubaneswar
Dt. 10/04/2018
We gathered in our college campus around 12 noon on 29/03/2018 and left for OPTCL in
around 12:30 pm. Since OPTCL is present very near to our college, so we reached there in five
minutes.
After reaching there we headed to their conference hall where Mr. KANHU CHARAN
BISOYI took an interactive class from 1:00pm to 3:30pm. In that teaching session he taught us
various things related to the generation of power from the generating station and the various
form of losses and their compensation. Then he taught us about the transmission and the
distribution system. Then he covered some basic components of transmission system such as the
conductors, transmission towers, etc and also covered some basic components of distribution
systems such as the surge arresters, circuit breakers, current transformer, wave trapper, isolators
etc. He also gave us a little knowledge about the various accidents that took place while working
the grid. He also provided us the various safety tips and precautions that should be taken during
the field visit in the grid.
After the interactive class we had some refreshments and around 3:30 pm we headed for
the field visit in the grid.
Before stepping in to the grid we were provided with the safety gears. Then we saw the
various components that are described below briefly:-
1. Capacitor Voltage Transformer (CVT)
2. Current Transformer (CT)
3. Wave Trap
4. Isolators
5. Circuit Breakers
6. Surge arrester
7. Purpose of guard ring in suspension insulators

6. ELECTRICAL POWER TRANSMISSION
Electric power transmission is the bulk movement of
electrical energy from a generating site, to an electrical
substation. The interconnected lines which facilitate this
movement are known as a transmission network. This is
distinct from the local wiring between high-voltage substations
and customers, which is typically referred to as electric power
distribution. The combined transmission and distribution
network is known as the "power grid"
POWER SUPPLY SYSTEM
Most transmission lines are high-voltage three-phase alternating current (AC), although
single phase AC is sometimes used in railway electrification systems. High-voltage direct-
current (HVDC) technology is used for
greater efficiency over very long distances
(typically hundreds of miles). HVDC links
are used to stabilize large power distribution
networks where sudden new loads, or
blackouts, in one part of a network can result
in synchronization problems and cascading
failures.
Electricity is transmitted at high voltages
(115 kV or above) to reduce the energy loss
which occurs in long-distance transmission.
Power is usually transmitted through overhead power lines. Underground power transmission has
a significantly higher installation cost and greater operational limitations, but reduced
maintenance costs. Underground transmission is sometimes used in urban areas or
environmentally sensitive locations.
OVER HEAD TRANSMISSION
High-voltage overhead conductors are not covered by insulation. The conductor material is
nearly always an aluminum alloy, made into several strands and possibly reinforced with steel
strands. Copper was sometimes used for overhead transmission, but aluminum is lighter, yields
only marginally reduced performance and cost much less. Improved conductor material and
shapes are regularly used to allow increased capacity and modernize transmission circuits.
Conductor sizes range from 12 mm2
to 750 mm2
, with varying resistance and current-carrying
capacity. Thicker wires would lead to a relatively small increase in capacity due to the skin effect
(which causes most of the current to flow close to the surface of the wire). Because of this
current limitation, multiple parallel cables (called bundle conductors) are used when higher

7. capacity is needed. Bundle conductors are also used at high voltages to reduce energy loss
caused by corona discharge
Today, transmission-level voltages are usually considered to be
110 kV and above. Lower voltages, such as 66 kV and 33 kV, are usually
considered sub transmission voltages, but are occasionally used on long
lines with light loads. Voltages less than 33 kV are usually used for
distribution. Voltages above 765 kV are considered extra high voltage and
require different designs compared to equipment used at lower voltages.
THE EFFECT OF LIGHTNING:
A direct lightning strike on a conductor of a power line causes extremely
high voltage pulses at the strike point, which are propagated as traveling
waves in either direction from the point of strike. The crest of the pulse
can be calculated as:
V = I × Z
Where:
V is the crest voltage
I is the peak lightning current
Z is the impedance seen by the pulse along the direction of travel.
Impedance Z is equal to half the surge impedance of the line when struck
at mid-point and can be approximately as much as 150 Ω. Thus for a peak current of 40 kA, the
voltage of the pulse can be as high as 6000 kV. Since the basic insulation level of most systems
is much lower than this value, it is clear that such a pulse will cause failure of insulating
components along the line.
It is therefore necessary that no direct strike must be permitted on the overhead power lines
phase conductors.
CAPACITOR VOLTAGE
TRANSFORMER (CVT)
A capacitor voltage transformer (CVT), is
a transformer used to step down extra high
voltage signals and provide a low voltage
signal, for metering or operating a protective
relay.
In its most basic form, it consists of three parts:
two capacitors, across which the transmission
line signal is split, an inductive element to tune
the device to the line frequency, and a voltage
transformer to isolate and further step down the voltage for metering devices or protective relay.
The tuning of the divider to the line frequency makes the overall division ratio less sensitive to
changes in the burden of the connected metering or protection devices. The device has at least

8. four terminals: a terminal for connection to the high voltage signal, a ground terminal, and two
secondary terminals which connect to the instrumentation or protective relay.
CVTs in combination with wave traps are used for filtering high-frequency communication
signals from power frequency. This forms a carrier communication network throughout the
transmission network, to communicate between substations. The CVT is installed at a point after
Lightning Arrester and before Wave trap.
CURRENT TRANSFORMER
A current transformer (CT) is a type of transformer that is used to measure alternating
current (AC). It produces a current in its secondary which is proportional to the current in its
primary.
CTs, potential transformers, are instrument transformers. Instrument transformers scale the
large values of voltage or current too small, standardized values that are easy to handle for
instruments and protective relays. The instrument transformers isolate measurement or protection
circuits from the high voltage of the primary system. A CT provides a secondary current that is
accurately proportional to the current flowing in its primary. The CT presents a negligible load to
the primary circuit.
CTs are the current-sensing units of the power system and are used at generating stations,
electrical substations, and in industrial and commercial electric power distribution.
CTs are used extensively for measuring current and monitoring
the operation of the power grid. Along with voltage leads,
revenue-grade CTs drive the electrical utility's watt-hour meter on
virtually every building with three-phase service and single-phase
services greater than 200 amperes.
High-voltage CTs are mounted on porcelain or polymer
insulators to isolate them from ground. Some CT configurations
slip around the bushing of a high-voltage transformer or circuit
breaker, which automatically centers the conductor inside the CT
window.
CTs can be mounted on the low voltage or high voltage leads of a
power transformer. Sometimes a section of a bus bar can be
removed to replace a CT.
The burden impedance should not exceed the specified
maximum value to avoid the secondary voltage exceeding the
limits for the CT. The primary current rating of a CT should not
be exceeded or the core may enter its non linear region and ultimately saturate.
CTs are often used to monitor high currents or currents at high voltages. Technical standards and
design practices are used to ensure the safety of installations using CTs.
The secondary of a CT should not be disconnected from its burden while current is in the
primary, as the secondary will attempt to continue driving current into an effective infinite

9. impedance up to its insulation break-down voltage and thus compromise operator safety. For
certain CTs, this voltage may reach several kilovolts and may cause arcing. Exceeding the
secondary voltage may also degrade the accuracy of the transformer or destroy it. Energizing a
CT with an open circuit secondary is equivalent to energizing transformer with secondary short
circuited. In the first case the secondary tries to produce an infinite voltage and in the second
case the secondary tries to produce an infinite current. Both scenarios can be dangerous and
damage the transformer.
CTs are used for protection, measurement and control in high-voltage electrical substations
and the electrical grid. CTs may be installed inside switchgear or in apparatus bushings, but very
often free-standing outdoor CTs are used. In a switchyard, live tank CTs have a substantial part
of their enclosure energized at the line voltage and must be mounted on insulators.
A HV CT may contain several cores, each with a secondary winding, for different purposes
(such as metering circuits, control, or protection). A neutral CT is used as earth fault protection
to measure any fault current flowing through the neutral line from the wye neutral point of a
transformer.
WAVE TRAP
A wave trap (high-frequency stopper) is a maintenance-free parallel resonant circuit,
mounted inline on high-voltage (HV) AC transmission power lines to prevent the transmission of
high frequency (40-1000 kHz) carrier signals of power line communication to unwanted
destinations. Line traps are cylinder-like structures connected in series with HV transmission
lines. A line trap is also called a wave trap.
The wave trap acts as a barrier or filter to prevent signal losses. The inductive reactance of
the line trap presents a high reactance to high-frequency signals but a low reactance to mains
frequency. This prevents carrier signals from being dissipated in the substation or in a tap line or
branch of the main transmission path and grounds in the case of anything happening outside of
the carrier transmission path. The line trap is also used to
attenuate the shunting effects of high-voltage lines.
The trap consists of three major components: the main coil,
the tuning device, and the protective device (also known as a
surge arrester). The protective and tuning devices are mounted
inside the main coil. A line trap may be covered with a bird
barrier, in which case there are four components.
The main coil is the outer part of the line trap which is made
from stranded aluminum cable. The reactor coil, depending on
the device, can be made up of several aluminum wires, allowing
equal distribution amongst the parallel wires. The coil carries
rated continuous power frequency currents; therefore this is the
power inductor in this system. It provides a low impedance path
for the electricity flow. Since the power flow is rather large at
times, the coil used in a line trap must be large in terms of
physical size. Hence, a line trap unit is inserted between the bus
bar and connection of coupling capacitor to the line. It has low
impedance for power frequency and high impedance to carrier frequency. This unit prevents the
high frequency carrier signal from entering the neighboring line.

10. The next major component is the tuning device. This device is securely installed inside the
main coil. It adjusts blocking frequency or bandwidth, and consists of coils, capacitors, and
resistors. With the use of a tuning device, a line trap can be tuned to a frequency of 1 kHz.
The last main component is the protective device, which is parallel with the main coil and
the tuning device. It protects the main coil and the tuning device by lowering the over-voltage
levels.
Wave traps are connected in series with power line and thus their coils are rated to carry the full
line current. The impedance of a line trap is very low at the power frequency and will not cause
any significant voltage drop.
High frequency line traps have a temperature limit of 115 °C-180 °C depending on construction
and manufacture.
ISOLATOR
An isolator (also known as disconnector or disconnect switch) is used to ensure that an
electrical circuit is completely de-energized for service or maintenance. Such switches are often
found in electrical distribution and industrial applications, where machinery must have its source
of driving power removed for adjustment or
repair.
High-voltage isolation switches are used in
electrical substations to allow isolation of
apparatus such as circuit breakers, transformers,
and transmission lines, for maintenance. The
isolator is usually not intended for normal
control of the circuit, but only for safety
isolation. Isolators (or disconnectors) can be
operated either manually or automatically.
Unlike load switches and circuit breakers,
disconnectors lack a mechanism for suppression of electric arcs, which occurs when conductors
carrying high currents are electrically interrupted. Thus, they are off-load devices, intended to be
opened only after current has been interrupted by some other control device. Safety regulations
of the utility must prevent any attempt to open the disconnector while it supplies a circuit.
Standards in some countries for safety may require either local motor isolators or lockable
overloads (which can be padlocked).
Disconnectors have provisions for a lockout-tagout so that inadvertent operation is not
possible. In high-voltage or complex systems, these locks may be part of a trapped-key interlock
system to ensure proper sequence of operation. In some designs, the isolator switch has the
additional ability to earth the isolated circuit thereby providing additional safety. Such an
arrangement would apply to circuits which inter-connect power distribution systems where both
ends of the circuit need to be isolated.
A switch disconnector combines the properties of the disconnector and the load switch, so it
provides the safety isolation function while being able to make and break nominal currents.

11. CIRCUIT BREAKERS
Electrical power transmission networks are protected and controlled by high-voltage
breakers. The definition of high voltage varies but in power transmission work is usually thought
to be 72.5 kV or higher, according to a recent definition by the International Electrotechnical
Commission (IEC). High-voltage breakers are nearly always solenoid-operated, with current
sensing protective relays operated through current transformers. In substations the protective
relay scheme can be complex, protecting equipment and buses from various types of overload or
ground/earth fault.
High-voltage breakers are broadly classified by the medium used to extinguish the arc:
i. Bulk oil iv. Vacuum
ii. Minimum oil v. Air blast
iii. SF6 vi.CO2
Due to environmental and cost concerns over insulating oil spills, most new breakers use SF6 gas
to quench the arc.
High-voltage circuit breakers used on transmission systems may be arranged to allow a single
pole of a three-phase line to trip, instead of tripping all three poles; for some classes of faults this
improves the system stability and availability.
SULFUR HEXAFLUORIDE (SF6) HIGH-VOLTAGE CIRCUIT BREAKERS
A SF6 circuit breaker uses contacts surrounded by sulfur hexafluoride gas to quench the arc.
They are most often used for transmission-level voltages and may be incorporated into compact
gas-insulated switchgear. In cold climates, supplemental heating or de-rating of the circuit
breakers may be required due to liquefaction of the SF6 gas.
SF6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas. The
circuit breakers may be connected into the circuit by bolted connections to bus bars or wires,
especially in outdoor switchyards. The circuit breakers in switchgear line-ups are often built with
draw-out construction, allowing breaker removal without disturbing power circuit connections,
using a motor-operated or hand-cranked mechanism to separate the breaker from its enclosure.
Some important manufacturer of VCB from 3.3 kV to 38 kV are ABB, Eaton, Siemens,
HHI(Hyundai Heavy Industry), S&C Electric Company, Jyoti and BHEL.
CARBON DIOXIDE (CO2) HIGH-VOLTAGE CIRCUIT BREAKERS
In 2012 ABB presented a 75 kV high-voltage breaker that uses carbon dioxide as the
medium to extinguish the arc. The carbon dioxide breaker works on the same principles as an
SF6 breaker and can also be produced as a disconnecting circuit breaker. By switching from SF6
to CO2 it is possible to reduce the CO2 emissions by 10 tons during the product’s life cycle.

12. PURPOSE OF GUARD RING IN SUSPENSION INSULATOR
When Suspension type insulator is connected to
hold a power conductor carrying electrical power at
high voltage, a charging current will flow though the
series connected Self Capacitors. As the charging
current through each of the Capacitor (Porcelain
disc) is same, therefore the potential distribution
across each Porcelain disc will be same i.e. each
Porcelain disc will have equal voltage stress.
But actually, in addition to self capacitance of
porcelain disc there also exists capacitance in
between the metallic link of suspension insulator and
grounded tower body. This capacitance is known as
Shunt Capacitance. Now due to this Shunt Capacitance, the charging current through each
porcelain disc will no longer be same rather it will decrease as we move from the disc nearer to
the power conductor to the disc farthest from the power conductor. Thus the voltage distribution
across the discs of Suspension Insulator is not uniform due to Shunt Capacitance.
Here comes the Grading Ring or Guard Ring. Grading Ring or Guard Ring equalizes the
potential distribution across each disc in Suspension Insulator. Grading Ring nullifies the effect
of shunt capacitance of string insulator.
As the non uniformity in potential distribution across the various discs are because of
current flowing in shunt capacitance, therefore if it is possible to supply the amount of current in
shunt capacitor, then throughout the string same current will flow. Because of Grading Ring,
capacitance between the Grading Ring and metallic link of disc forms. It is constructed in such a
way that shunt capacitance currents are equal to the metal fitting line capacitance current.
Because of this the charging current flowing through the whole string of discs will be same due
to which a uniform potential distribution is achieved.
SURGE ARRESTER
A surge arrester is a device to protect electrical equipment from over-voltage transients
caused by external (lightning) or internal (switching) events. Also called a surge protection
device (SPD) or transient voltage surge suppressor (TVSS), this class of device is used to protect
equipment in power transmission and distribution systems. The energy criterion for various
insulation materials can be compared by impulse ratio, the surge arrester should have a low
impulse ratio, so that a surge incident on the surge arrester may be bypassed to the ground
instead of passing through the apparatus.
To protect a unit of equipment from transients occurring on an attached conductor, a surge
arrester is connected to the conductor just before it enters the equipment. The surge arrester is
also connected to ground and functions by routing energy from an over-voltage transient to
ground if one occurs, while isolating the conductor from ground at normal operating voltages.
This is usually achieved through use of a varistor, which has substantially different resistances at
different voltages.

13. Surge arresters are not generally designed to protect against a direct lightning strike to a
conductor, but rather against electrical transients resulting from lightning strikes occurring in the
vicinity of the conductor. The lightning which strikes the earth results in ground currents which
can pass over buried conductors and induce a transient that propagates outward towards the ends
of the conductor. Surge arresters only protect against induced transients characteristic of a
lightning discharge's rapid rise-time and will not protect against electrification caused by a direct
strike to the conductor. Transients similar to lightning-induced, such as from a high voltage
system's fault switching, may also be safely diverted to ground; however, continuous over
currents are not protected against by these devices. The energy in a handled transient is
substantially less than that of a lightning discharge; however it is still of sufficient quantity to
cause equipment damage and often requires protection.
LIGHTNING ARRESTER
A lightning arrester is a device used on electric power systems to protect the insulation and
conductors of the system from the damaging effects of lightning. The typical lightning arrester
has a high voltage terminal and a ground terminal.
When a lightning surge travels along the power line to the arrester, the current from the surge is
diverted through the arrester, in most cases to earth. Smaller versions of lightning arresters, also
called surge protectors, are devices that are connected between each electrical conductor in
power and communications systems and the Earth. These prevent the flow of the normal power
or signal currents to ground, but provide a path over which high-voltage lightning current flows,
bypassing the connected equipment. Their purpose is to limit the rise in voltage when a
communications or power line is struck by lightning or is near to a lightning strike.
If protection fails or is absent, the lightning that strikes the electrical system introduces
thousands of kilovolts that may damage the transmission lines, and can also cause severe damage
to transformers and other electrical or electronic devices. Lightning-produced extreme voltage
spikes in incoming power lines can damage electrical home appliances or even produce death.
Lightning arresters are used to protect electric fences. They consist of a spark gap and sometimes
a series inductor.